This renewal application for the P30 Center for Drug Use and HIV Research (CDUHR) focuses on An Interdisciplinary Approach to HIV and Other Infectious Diseases of Drug Users. The current Center emphasizes socio-behavioral factors influencing HIV prevention and transmission and has made important contributions to the field. We now propose to expand our scope of research, to a wider perspective on clinical infectious diseases (e.g., sexually transmitted infections [STIs] and Hepatitis C [HCV]) in drug users and their communities and strengthen our interdisciplinary focus by integrating clinical infectious diseases with socio-behavioral-epidemiologic research. Based on emerging research needs in the field regarding drug users, we will focus more on (1) sexual HIV transmission; (2) HCV and HIV/HCV co-infection; (3) service access disparities; and (4) understudied topics in domestic and international HIV research, including geographic disparities in infection, innovative interventions to address sexual transmission, and learning how social and behavioral processes interact with epidemiology and pathogen characteristics. We will also enhance dissemination of findings and collaborations among community groups, clinical infectious disease specialists and socio-behavioral researchers. The over-arching aim of the Center will be to enhance interdisciplinary research on HIV and related infectious diseases among drug users. CDUHR will also serve as a resource to disseminate emerging research methods and findings. The Center will provide a research infrastructure for training and consultation to increase productivity and collaboration across investigators, enhance research efficiency and create synergy leading to the development of new investigators, new initiatives and new knowledge in interdisciplinary research. CDUHR-III will achieve these aims through 5 Cores (1) Infectious Diseases - serves as a resource regarding clinical infectious diseases of drug users and related health care issues; (2) Theoretical Synthesis - integrates theories drawn from diverse disciplines; (3) Interdisciplinary Research Methods - fosters excellence in study design and analysis; (4) Dissemination develops dissemination mechanisms for service provider audiences; and (5) Administration - provides scientific leadership and ensuring collaboration and synergy across Cores, projects and investigators.